Longitudinal PET Imaging of Antipsychotic Binding to the Dopamine-3 Receptor in Schizophrenia

There is a great need for the development of new treatments for schizophrenia (SCZ). Aside from the recently approved muscarinic agonist xanomeline, all current treatments have been assumed to function by blocking dopamine-2 receptors (D2Rs). Interest in the dopamine-3 receptor (D3R) was encouraged by preclinical findings that D3R antagonists reverse cognitive impairment and improve negative symptoms. In vivo imaging of the D3R became possible with the development of \[11C\]-(+)-PHNO, a D3R-preferring radioligand. However, while numerous preclinical studies have demonstrated that the D3R is relevant to both the neurobiology and treatment of SCZ, in vivo studies initially and surprisingly reported that, after several weeks of administration, antipsychotic medications may not bind to the D3R and may paradoxically increase levels of the D3R. These findings were discrepant with our own findings in non-human primates and individuals with SCZ demonstrating that acute doses of antipsychotic medications bind to the D3R and D2R in ratios predicted by their in vitro binding profiles. In a later study conducted by our group, 10 days of chronic dosing of the D2R-preferring antipsychotic medication brexpiprazole (BREX) also led to increased levels of the D3R at 1mg and negligible binding at 4mg. Finally, in our study of the D3R-preferring antipsychotic medication cariprazine (CAR), there was robust binding to the D3R and D2R after both acute and subchronic dosing. These seemingly discrepant findings may be related to methodological differences, differences in the binding profiles of D2R- vs. D3R-preferring antipsychotic medications, or to homeostatic responses to chronic antipsychotic treatment (i.e., upregulation). Upregulation is a potentially critical, though underexamined, effect common to all D2R/D3R-binding antipsychotic medications, including partial agonists. It has been hypothesized to be largely responsible for differences in occupancy estimates from single and repeat dose studies and contribute to waning effects of antipsychotic medications and tardive dyskinesia. The goals of this proposal are to elucidate the contribution of D3R, compared to D2R, binding to antipsychotic action, both acutely (SA1) and subchronically (SA2), in the same patients. The relationship between occupancy, upregulation, and clinical effects (SA3, EA) will be investigated.